ABSTRACT ? PROJECT 1 Plexiform neurofibromas (pNF) are complex nerve and soft tissue tumors that affect 25-50% of people with NF1. These tumors cause lifelong, progressive morbidity ranging from deformity to paralysis and death. Plexiform neurofibromas are clinically challenging because they involve multiple body regions and encompass critical portions of the nervous system making surgical excision unfeasible. Moreover, they are multicellular, comprised of Schwann cells, blood vessels, fibroblasts and immune cells requiring informed strategies to identify effective drug therapies. In a series of genetic and bone marrow transplantation studies in genetically engineered mice (GEM), the Clapp and Parada groups demonstrated that the Nf1 GEM is relevant to human pNF pathophysiology, that there is a complex interplay between Nf1-/- Schwann cells and the microenvironment, and that targeting this interaction reduces tumor burden in mice. The phase 2 study of imatinib showed impressive tumor response in a subset of patients, however, the majority of tumors did not respond (Robertson et al, Lancet Oncology, 2012). Recent genetic and pharmacologic studies identify the Ras-Raf-Erk pathway as key in NF1 mediated tumors and suggest that Mek inhibition is another important therapeutic strategy for pNF. However, similar to imatinib, there is variable tumor response in both GEM and patients. This raises the question: what factors mediate variable responses in pNF? In order to determine the factors that mediates the variable responses, our group will accomplish the following: (1) explore adaptive responses to Mek inhibition via RNAseq and kinome studies in pre- and post-treatment tumor tissue in patients with pNF treated with the Mek inhibitor selumetinib and investigate circulating hematopoietic stem/progenitor cells before, during and after treatment in patients; (2) evaluate the relationship between treatment response and tumor factors (location, age, and cellular, protein and genetic features before and after treatment) across GEM and patients with NF1; (3) investigate the clinical, molecular and pharmacokinetic (PK) effects of combined c-kit and Mek inhibition in GEM; (4) explore adaptive responses to Mek and c-kit inhibition alone and in combination in GEM via quantitative proteomics, transcriptomics (RNAseq) and exome sequencing in collaboration with Dr. Johnson in the Omics core and (5) investigate the therapeutic window(s) for c-kit inhibition at distinct embryonic and adult stages of pNF formation in GEM.